Gas turbine engines include a multistage axial compressor that pressurizes air, mixes the pressurized air with fuel, and ignites the compressed air/fuel mixture to generate hot combustion gases that flow downstream through a high pressure turbine, which extracts useful energy therefrom. Each compressor stage usually includes a row of compressor rotor blades extending radially outwardly from a supporting rotor hub. Each blade includes an airfoil over which the air being pressurized flows.
The high speed with which the compressor hub rotates during operation generates very large centrifugal forces that stress the rotor blades. Over time, the stresses can damage the rotor blades, requiring them to be replaced. Accordingly, the rotor blades are usually designed to be removable so they can be replaced without replacing the hub or other parts of the turbine engine. For example, rotor blades typically have a root beneath with a dovetail configured to engage a complementary dovetail slot in the perimeter of the rotor hub. The dovetail has pressure faces that engage corresponding inner surfaces of the slot to retain the blade in the slot against the outward centrifugal force generated by the rotating hub. Typically, the dovetails are either axial-entry dovetails, which engage the slot in the direction of the axis of the turbine engine, or circumferential-entry dovetails, which engage the slot in the direction perpendicular to the axis of the turbine engine.
Techniques have been developed to prolong the useful life of the rotor hub and/or of the rotor blades themselves. One such technique is described in U.S. Pat. No. 6,033,185 to Lammas et al., issued on Mar. 7, 2000 (the '185 patent). According to the '185 patent, the maximum dovetail stress may be initially found at the dovetail neck in early blade life, but then transitions to the outer edges of the pressure faces at mid-life. The '185 patent states that this mid-life transition in maximum stress can lead to a shortening in remaining available life of the blade dovetails.
To purportedly address this problem, the '185 patent proposes a circumferentially-mounted rotor blade that includes undercuts in the pressure faces of the dovetail lobe. According to the '185 patent, the undercuts introduce a stress concentration in the neck of the rotor blade that initially increases the maximum stress experienced at outer edges of the pressure faces of the blade dovetail in early life (before the dry lubricant fails), but significantly reduces the maximum stress which would otherwise occur as the dry lubricant wears in operation beyond mid-life. The '185 patent explains that this tradeoff increases the overall life of the rotor blade. An undercut similar to the '185 patent undercut is also disclosed in S. J. Shaffer et al., Fretting Fatigue, ASM Handbook, Volume 19 (1996).